1. Field of the Invention
The present invention relates in general to a multiple level voltage generator for a semiconductor memory device, and more particularly to a multiple level voltage generator in which a single ring oscillator is used instead of a plurality of ring oscillators to drive a plurality of charge pumping circuits, so that power consumption can be reduced.
2. Description of the Prior Art
Generally, a back bias voltage generator and a high voltage generator are adapted to generate a low back bias voltage and a bootstrapped high voltage, respectively, which are used in a semiconductor memory device. The back bias voltage is applied to a well or a substrate to maintain a junction between the well or substrate and a general circuit at a reverse bias state, so as to separate them from each other. The bootstrapped high voltage is used to increase the current drive capability of an NMOS transistor and prevent an output voltage loss.
In a conventional multiple level voltage generator for a semiconductor memory device, a plurality of oscillators are provided to generate pulses to drive a plurality of charge pumping circuits. As the oscillators are increased in number, current consumption is increased, too. In particular, oscillators for the generation of back bias and high voltages are very large in current consumption.